1. Field of the Invention
The present invention relates to a laser module in which at least one laser beam emitted from at least one semiconductor laser element is coupled to an optical fiber.
2. Description of the Related Art
Conventionally, the laser modules known as the pigtail-type laser modules have been widely used in the field of optical communications. The pigtail-type laser modules are constituted by a semiconductor laser element contained in a package, an optical fiber being fixed to the package and having an end (light-entrance end) which appears inside the package, and an optical condensing system which couples a laser beam emitted from the semiconductor laser element with the optical fiber at the light-entrance end face.
As a variation of the above pigtail-type laser modules, the combined-laser modules are also known, for example, as indicated in Japanese Unexamined Patent Publication No. 2003-298170. In the combined-laser modules, a plurality of laser beams are emitted from one or more semiconductor laser elements, and enter an optical fiber which is arranged as in the pigtail-type laser modules, so that the plurality of laser beams are combined into a single laser beam having high intensity.
In the above laser modules, in order to stably maintain the state in which the semiconductor laser and the light-entrance end face of the optical fiber are optically coupled with a precision on the order of micrometers, the optical fiber, the optical condensing system, and the like are normally fixed by using a means of adhesion such as a solder or an adhesive.
In addition, in the laser modules for communications, in order to prevent deterioration of the laser caused by moisture in the atmosphere and the like, normally the package is hermetically sealed. The so-called CAN package is a typical sealing structure which protects semiconductor laser elements and laser end facets. Japanese Unexamined Patent Publication No. 2003-298170 also discloses hermetic sealing of a package containing the aforementioned one or more semiconductor laser elements and optical fiber, an optical condensing system, and the like.
In the laser modules as mentioned above, contaminants remaining in the hermetically sealed package are likely to be deposited on the light-emission end face of the semiconductor laser element and other optical components such as the optical condensing system and the optical fiber, so that the laser characteristics deteriorate. In particular, in the regions through which light passes and in which the optical density is high, the effect of depositing materials (i.e., the dust collection effect) is enhanced. Further, in the laser modules containing semiconductor laser elements which emit laser beams in the wavelength range of 350 to 500 nm (e.g., in the 400 nm band) such as GaN semiconductor laser elements, the photon energy is high. Therefore, photochemical reactions with the deposited materials are likely to occur, and the dust collection effect becomes further enhanced.
Typical examples of the contaminants are hydrocarbon compounds which enter the package from ambient atmospheres during manufacturing processes. The laser light polymerizes or decomposes such hydrocarbon compounds, and the materials produced by the polymerization or decomposition are deposited and impede increase in the optical output power.
In addition, it is known that ultraviolet light causes photochemical reactions with low-molecular-weight siloxane suspended in air, and the reaction products SiOx are deposited on optical glass window elements. Therefore, periodic replacement of the window elements exposed to air is recommended, for example, as indicated in Japanese Unexamined Patent Publication No. 11 (1999)-054852.
Various proposals have been made for suppressing the aforementioned dust collection effect. For example, U.S. Pat. No. 5,392,305 indicates a proposal to mix 100 ppm or higher of oxygen into sealing gas for decomposing the hydrocarbon compounds and the like.
Further, in optical systems in which ultraviolet light having a wavelength of 400 nm or smaller is applied to optical elements, arrangement of the optical elements in an atmosphere containing 99.9% or more nitrogen has been proposed, for example, as indicated in Japanese Unexamined Patent Publication No. 11 (1999)-167132.
Furthermore, it is known that the dust collection effect can be suppressed by degassing the inside of the package immediately before hermetically sealing the package.
However, in the case where a laser module in which a commercially available optical fiber covered with a primary coating of an ultraviolet-light-curing resin and a secondary coating of a polymer is fixed to a package, degassing is performed after the optical fiber is fixed to the package. That is, the fiber coatings exist in a degassing system. Therefore, chemical components are outgassed during the degassing operation, and thus the inside of the laser module is contaminated with the outgassed chemical components. In order to prevent this contamination, all the coatings of the optical fiber may be removed in advance. Nevertheless, since the optical fiber without coatings is prone to break, and it is difficult to handle an optical fiber without coatings, the removal of the coatings is not practical.
As indicated in Japanese Unexamined Patent Publication No. 2003-298170, hermetic sealing of a package containing a semiconductor laser element, an optical fiber, an optical condensing system, and the like is effective at preventing contamination of the inside of the laser module. However, in this case, the volume of the package becomes considerably large. When the volume of a hermetically sealed package is increased, the cost of the package is also increased, and assembly becomes more difficult. Therefore, in the case where a laser module is produced by hermetically sealing a large package containing a semiconductor laser element, an optical fiber, an optical condensing system, and the like, the cost of the laser module becomes very high.